Context: I’m building an Echo schema that embeds_one schema which itself contains an embeds_many field. I need to allow users to add and remove from the embeds_many field. I followed the tutorial provided in the Phoenix.Component.inputs_for/1 documentation hoping to take advantage of Ecto.Changeset.cast_embed’s :sort_param/:drop_param.

Problem: When embeds are cast, the drop param gets treated as a new embedded entity and gets passed to changeset for validation. Of course this crashes every time the form is modified as drop param is a list of indexes and not a map of user input data. The docs linked above state:

Outside the inputs_for , we render an empty mailing_list[emails_drop][] input, to ensure that all children are deleted when saving a form where the user dropped all entries. This hidden input is required whenever dropping associations.

In my case since this is an embeds_many inside of an embeds_one, my empty input looks like this patron[menu_item][modifications][mods_drop][]. When the empty form is validated the value [""] gets passed to my changeset function. When delete is pressed on a populated form, the index gets passed to the changeset function like so ["1"] (populated in mount, can’t edit form in UI as it crashes on every change).

The data passed to LiveView on change looks like so:

%{
  "_persistent_id" => "0",
  "item_name" => "",
  "modifications" => %{
    "0" => %{
      "_persistent_id" => "0",
      "comment" => ""
    },
    "mods_drop" => [""],
    "mods_sort" => ["0"]
  }
}

If mods_drop => ["3"] then ["3"] gets passed to the modifications changeset. If I comment out the hidden mods_drop input, the same problem occurs, this time on the mods_sort input. I feel I must be missing something crucial in the docs but I’m not sure what.